A method is known, which comprises, when manufacturing steel from steel scrap in an electric furnace, preheating the steel scrap by using a high-temperature exhaust gas produced in the electric furnace, and charging the steel scrap thus preheated into the electric furnace for refining. According to this method, it is possible to reduce the refining time in the electric furnace, and to save the electric power consumption required for refining.
FIG. 1 is a schematic drawing illustrating an embodiment of the conventional method for preheating steel scrap by an exhaust gas from a steelmaking electric furnace. In FIG. 1, 1 is an electric furnace; 2 is a discharging duct of an exhaust gas produced in the electric furnace 1; 4 is an exhaust gas combustion chamber provided in the middle of the discharge duct 2; and, 6 is a scrap preheating chamber provided in the middle of a branch duct 7 branching off from the discharge duct 2. When manufacturing steel from steel scrap in the electric furnace 1, an exhaust gas produced in the electric furnace 1 is discharged through an exhaust duct 5 provided in the furnace lid of the electric furnace 1, and directed to the exhaust gas combustion chamber 4 provided in the middle of the discharge duct 2 together with air in an appropriate amount sucked from a gap 3 having an adjustable opening provided at an end of the exhaust duct 5 to cause combustion of carbon monoxide contained in the exhaust gas and thus to form a combustion exhaust gas.
The combustion exhaust gas is directed to a scrap preheating chamber 6 provided in the middle of the branch duct 7 branching off from the exhaust duct 2, and preheats the steel scrap charged in the scrap preheating chamber 6 to a prescribed temperature. The combustion exhaust gas after preheating the steel scrap is discharged to the open air from a chimney 11 through a cooling chamber 8 and a dust collector 9 provided on the discharge duct 2. 10 is a fan provided in the middle of the discharge duct 2; 14 is another fan provided in the middle of the branch duct 7; and, 12 and 13 are dampers.
FIG. 2 is a schematic drawing illustrating another embodiment of the conventional method for preheating steel scrap. In this embodiment, two scrap preheating chambers 6 and 6' are provided in parallel with each other in the middle of the branch duct 7 branching off from the discharge duct 2; dampers 12 and 12' are provided on the branch duct 7 on the combustion exhaust gas entry side of the scrap preheating chambers 6 and 6'; and, dampers 13 and 13' are provided on the branch duct 7 on the combustion exhaust gas exit side of the scrap preheating chambers 6 and 6'. By opening one of the dampers 12 and 12' and closing the other thereof, the combustion exhaust gas is directed to only one of the two scrap preheating chambers 6 and 6' to preheat steel scrap charged in this scrap preheating chamber. It is therefore possible to prepare for the next run of preheating by charging another batch of steel scrap, which is to be preheated, into the other scrap preheating chamber.
Steel scrap usually contains entrapped oil, rubber, vinyl, plastics and other combustible substances. Therefore, when preheating steel scrap charged in a scrap preheating chamber by a high-temperature combustion exhaust gas, these combustible substances entrapped in the steel scrap burn in contact with the combustion exhaust gas under the influence of the heat thereof. This combustion, being an incomplete combustion in general, produces an incomplete-combustion gas containing hydrocarbon in the form of a white fume emitting an offensive odor and carbon monoxide. This incomplete-combustion gas, which cannot be caught by a dust collector and is therefore discharged to the open air as it is, forms a source of air pollution and this has been considered a problem impairing the environmental health.
The temperature of an exhaust gas produced in an electric furnace varies throughout the entire refining process from the beginning to the end of refining, not being kept at a constant level. However, according to the conventional method for preheating steel scrap, the exhaust gas has been fed into the scrap preheating chamber always in a constant quantity to preheat steel scrap. As a result, the steel scrap preheated in the scrap preheating chamber has varied in temperature according to the timing of production of the combustion exhaust gas in the electric furnace having preheated the steel scrap: it has sometimes been over the necessary level, or under this level in some others, being far from reaching the target preheating temperature.
When the temperature of the combustion exhaust gas directed to the scrap preheating chamber from the electric furnace through the exhaust gas combustion chamber is over the necessary level, combustion of the above-mentioned combustible substances entrapped in the steel scrap during preheating by the combustion exhaust gas becomes violent, thus causing oxidation of the steel scrap. This results in a lower steelmaking yield in the steelmaking operation in which the above-mentioned steel scrap is used as a raw material. The steel scrap, put in a prescribed basket, is charged into the scrap preheating chamber, and is preheated by the combustion exhaust gas blown into this basket. When the temperature of this combustion exhaust gas is over the necessary level, the above-mentioned basket is subjected to a thermal deformation by the high-temperature combustion exhaust gas. The steel scrap in the basket thermally expands to a larger volume under the effect of heating by the high-temperature combustion exhaust gas. As a result, the steel scrap does not smoothly drop from the basket when charging the steel scrap into the electric furnace, thus making it difficult to charge the scrap into the electric furnace. Even if the steel scrap is charged into the electric furnace, there may be the problem of the lid of the electric furnace which cannot be closed. When the temperature of the combustion exhaust gas directed to the scrap preheating chamber is low, on the other hand, it is impossible to preheat the steel scrap charged in the scrap preheating chamber to a prescribed temperature.
Under the above-mentioned circumstances, there is a strong demand for development of a method in which, in preheating steel scrap to be charged into an electric furnace by using an exhaust gas produced in the electric furnace, discharge of a white-fume-like incomplete-combustion gas emitting an offensive odor resulting from the incomplete combustion of such combustible substances as oil, rubber, vinyl and plastics entrapped in the steel scrap under the effect of the heat of the above-mentioned exhaust gas is prevented, and also preheating of the steel scrap always to a prescribed temperature is accomplished, even if the temperature of the exhaust gas for preheating the steel scrap varies. However, such a method has not as yet been proposed.